NCEES 2008 Structural Morning Questions

[maximus808]

Questions 109, 111, and 112 seem very specific and the CERM doesn't go into much detail of providing information regarding the answers for these. Should I be concerned on the exam?

Q109. What type of load prevents the greatest uplift force on a footing for a structure?

Ans. Dead load and Windload

Q111. What would be the effect of changing the yield strength of a steel beam from 50 ksi to 36 ksi while keeping the remaining design data the same?

Ans. Deflection will not change.

Q112. Assuming all steel areas are equal, which of the following steel sections is most efficient for use as a beam spanning 20 ft. with an unbraced length of the compression flange of 20 ft'?

Ans. A) Square Section

Other answers are I section, 7 (mirrored), C Section?

Thanks.

 

[bootlegend]

I think I know the questions you are referring to even though I don't have the book in front of me.

109

Maximum uplift will (in the case of a normal frame) always occur with maximum lateral load and minimum vertical load. DL+WL or DL+EL, etc.

111

Look at the deflection equations for beams. Yield stress is not a variable. Deflection depends only on E and I of the section. And loading, supports, etc of course.

112

Look into Lateral Torsional Buckling in your steel textbook. A square section is more stable (more torsionally strong) against lateral torsional buckling.

 

[Ble_PE]

Those questions are pretty basic structural analysis/mechanics of materials questions. You should have been exposed to these subjects in college, so go through the textbooks to study.

For question 109 you misstated what the problem was asking. It asked you to determine what load case governed the design for uplift on Footing A. Looking at the loaded frame, it should be obvious that the dead load + wind load combination would be the only combination to cause an uplift load on the footing.

For question 111, in you mechanics of materials class you should have learned that deflections are proportional to the inverse of the modulus of elasticity of the material and the moment of inertia. Most structural analysis textbooks have tables with beam deflection equations and you can see that the yield stress of the material never enters into the equations.

Question 112 is probably the most difficult question of all the above because it requires you to recognize that a beam with an unbraced flange over that length is susceptible to lateral torsional buckling. I won't get into the details of resisting torsion here, but just know that the best way to combat it is by providing material away from the centroid of the section. That means that tubes and pipe sections are the most efficient for carrying torsion.

Edit: Looks like bootlegend beat me to it.

 

[sab35263]

Those questions are pretty basic structural analysis/mechanics of materials questions. You should have been exposed to these subjects in college, so go through the textbooks to study.
For question 109 you misstated what the problem was asking. It asked you to determine what load case governed the design for uplift on Footing A. Looking at the loaded frame, it should be obvious that the dead load + wind load combination would be the only combination to cause an uplift load on the footing.

For question 111, in you mechanics of materials class you should have learned that deflections are proportional to the inverse of the modulus of elasticity of the material and the moment of inertia. Most structural analysis textbooks have tables with beam deflection equations and you can see that the yield stress of the material never enters into the equations.

Question 112 is probably the most difficult question of all the above because it requires you to recognize that a beam with an unbraced flange over that length is susceptible to lateral torsional buckling. I won't get into the details of resisting torsion here, but just know that the best way to combat it is by providing material away from the centroid of the section. That means that tubes and pipe sections are the most efficient for carrying torsion.

Edit: Looks like bootlegend beat me to it.

for question 112.

The tubes are more efficient because they have higher Inertia compared to the rest. Hope this helps.

 

[bootlegend]

sab,

I don't think that is necessarily true as a rule. Given the same area, a wide flange shape can have a higher moment of inertia than a tube. It depends on the distribution of the areas about the centroid. For example, a W8x24 has A=7.08 sq in and Ix=82.7in^4

A HSS8x8x1/4 has A=7.10 sq in and Ix=70.7 in^4.

 

[sab35263]

sab,
I don't think that is necessarily true as a rule. Given the same area, a wide flange shape can have a higher moment of inertia than a tube. It depends on the distribution of the areas about the centroid. For example, a W8x24 has A=7.08 sq in and Ix=82.7in^4

A HSS8x8x1/4 has A=7.10 sq in and Ix=70.7 in^4.

Hi,

Thanks for the response but I meant the Polar moment of inertia, in4 (J) is always higher in closed shapes such as tube compared to open section like I-beams.

To answer the question properly, the torsional shear stress on the open wide-flange section is MUCH LARGER than the closed sections! In fact, the open section experiences approximately 300 – 400 times the amount of stress than the closed sections! This is why CLOSED SECTIONS ARE MUCH MORE CAPABLE OF RESISTING TORSION THAN OPEN SECTIONS.

 

[bootlegend]

Okay. I assumed you meant Ix. My mistake.